1. Field of the Invention
The present invention relates to a porous membrane and a method for preparing the same, and a filter and a liquid treating module using the same. More particularly, the present invention is concerned with a porous membrane used as a porous membrane for blood treatment, a carrier membrane for specific adsorption and the like, and used for a bioreactor, an external circulatory curing device, various artificial organs and the like; and a method for preparing the same, and a filter and a liquid treating module using the same.
2 Description of the Prior Art
As a method for preparing a composite membrane material using a porous membrane as a substrate, it is customary to use a method which comprises coating a porous membrane substrate with a polymer other than one of which the porous membrane substrate is formed.
Heretofore, in a method for preparing a composite porous membrane of this type, as Japanese Patent Disclosure, SHO-62-163703 discloses, a polypropylene porous membrane substrate is prepared by a drawing method and then the substrate is coated with a polydiacetoneacrylamide to obtain a composite membrane. However, this method has drawbacks not only in that the void volume of the composite membrane is decreased by substantially the same volume as that of the coated polydiacetoneamide, thereby leading to a small pore size but also in that when the composite membrane is drawn in order to enlarge the pore size, since a polydiacetoneamide has a high glass transition temperature and the polypropylene porous membrane substrate per se has been prepared by a drawing method so that it is difficult to further sufficiently draw the composite membrane, the composite membrane is likely to be broken, Japanese Patent Disclosure, SHO-62-148667 also discloses a method for preparing a polypropylene porous membrane substrate. However, since a polypropylene porous membrane substrate prepared by this method has an elongation at break of about 10%, it is difficult to draw the polypropylene porous membrane substrate per se.
On the other hand, since hydrophobic porous membranes represented by polyolefin porous membranes have excellent mechanical strength and chemical resistance and do not undergo deformation due to swelling in an aqueous solvent, it is expected to use them in a wide variety of applications. However, as hydrophobic porous membranes are water repellent so that the hydrophobic porous membranes as such are not permeable to water, it is required to render them hydrophilic prior to use. As a method for rendering a hydrophobic porous membrane hydrophilic, there has been proposed a method which comprises forming polymerization starting points on a hydrophobic porous membrane by means of radiation, low temperature plasma, or the like, and supplying a hydrophilic monomer to form a graft polymer on the surface of the membrane (see Japanese Patent Disclosure, SHO-62-262705).
As mentioned above, conventional methods for preparing a porous composite membrane have drawbacks in that the pore size of a composite membrane is likely to become small, and that even if a composite membrane is subjected to drawing with the intention of obtaining a composite membrane having a large void volume and a large pore size, sufficient draw ratio can not be attained.
The above-mentioned method in which a graft polymer is formed on the surface of a membrane to render the membrane hydrophilic is excellent in inhibiting elution and imparting hydrophilic properties to the composite membrane. In the method, however, when a monomer described in Japanese Patent Disclosures, SHO-62-262705 and SHO-59-160504 such as acrylic acid, methacrylic acid or 2-hydroxyethyl methacrylate is used in to prepare a composite membrane, there is a practical problem that the resultant composite membrane is likely to be poor in mechanical strength. That is, physical properties of a particular structure having a high void volume and a small volume of a material portion of the structure, such as a porous membrane greatly vary depending upon the structure and characteristics of graft chains. Therefore, when a polymer having a high glass transition temperature is used to form graft chains on a porous membrane, the resultant porous membrane is brittle, thereby causing problems in handling such as damage in handling and lowering of pressure resistance.
Further, conventional porous composite membranes have a drawback in that when they are implanted in an organism as an artificial organ, they can not adapt their shape to the circumferential tissue so that stress concentrates on the junctional portion and sutural portion, thereby damaging the organism. On the other hand, for example, a polyurethane as a material for a porous membrane has a drawback in that although it has contraction and expansion properties to some extent, when it is implanted in an organism for a long period of time, it undergoes deterioration in the organism so that its physical properties are extremely impaired.